ABSTRACT: Drought stress at the reproductive stage causes severe
damage to productivity of wheat. However, little is known about the metabolites
associated with drought tolerance. The objectives of this study were to elucidate
changes in metabolite levels in wheat under drought, and to identify potential metabolites associated with drought stress through untargeted
metabolomic profiling using a liquid chromatography-high resolution mass spectrometry
(LC-HRMS)-based technique called Isotopic Ratio Outlier Analysis. Metabolomic
analysis was performed on flag leaves of drought-stressed and control (well-watered)
plants after 18 days of post-anthesis drought stress at three-hour intervals over
a 24-hour period. Out of 723 peaks detected in leaves, 221 were identified as known
metabolites. Sixty known metabolites were identified as important metabolites by
3 different methods, PLS-DA, RF and SAM. The most pronounced accumulation due to
drought stress was demonstrated by tryptophan, proline, pipecolate and linamarin,
whereas the most pronounced decrease was demonstrated by serine, trehalose, N-acetyl-glutamic
acid, DIBOA-glucoside etc. Three different patterns of metabolite accumulation were
observed over 24-hour period. The increased accumulated metabolites remained higher
during all 8 time points in drought stressed leaves. On the contrary, metabolites
that showed decreased level remained significantly lower during all or the most
time points. However, the levels of some decreased metabolites were lower during
the day, but higher during night in drought stressed leaves. Both univariate and
multivariate analyses predicted that N-acetyl-glutamic acid, proline, pipecolate,
linamarin, tryptophan, and DIBOA-glucoside could be potential metabolite biomarkers,
and their levels could serve as indicators of drought tolerance in wheat.